Thin films of materials such as semiconductors, superconductors, refractory materials, and many others, are of interest for a large variety of technological applications. The growth of thin films, typically by techniques such as chemical vapor deposition and molecular beam epitaxy, requires careful control of growth rates and reactant ratios, as film growth is usually dominated by the concentration of the active species. Precise control of these species is required for the reproducible growth of abrupt interfaces, constant composition layers, and graded junctions.
For example, precise knowledge and control of the relative partial pressures of metal-organics and hydrides is critical to the accurate and reproducible growth of epitaxial layers via metal-organic chemical vapor deposition (MOCVD). The current MOCVD growth technique requires calibration growth runs to determine reactant gas flow stabilization times and dwell times to flush the reactants and halt the growth process. However, subsequent runs may differ from the calibration runs due to unpredictable changes in flow conditions caused by faulty equipment or depleted metal-organic sources.
Reactant partial pressures are currently calculated from measurements of the metal-organic bubbler flow, metal-organic source temperature, back pressure over the source, carrier flow, and growth chamber pressure. A problem with using these calculations for on-line control is that they are based on several measurements which are difficult to accurately obtain.
Several in situ diagnostic techniques have been employed or proposed to provide information on the atmospheric conditions inside the growth chamber.
Coherent Antistokes Raman Spectroscopy (CARS) monitors the partial pressure of AsH.sub.3 concentration at a specific point in the reaction tube as a function of time. However, CARS requires two high quality, precisely aligned, lasers and, therefore, is not well suited for production operations.
Infrared absorption spectroscopy of metal-organics and hydrides utilizing an infrared laser diode with a multiple pass cell has been used to monitor partial pressures of the metal-organics and observe reaction products due to decomposition. The complexity of the cell design, the cost of the measurement apparatus, and the effects of the measurement on the growth environment preclude its widespread implementation as a diagnostic tool.
Techniques which are capable of monitoring the concentration of reactants in chemical vapor deposition reactors include mass spectroscopy, infrared diode laser spectroscopy, laser diagnostic probes, and ultraviolet (UV) absorption spectroscopy. It is this invention that recognizes the advantages of using UV absorption spectroscopy for reactant control.